1. Field of the Invention
The present invention relates to a radiation image information reader, and more particularly to a radiation image information reader for exciting a storable phosphor sheet storing radiation image information with line excitation light and then reading photostimulated luminescent light emitted from the sheet by line sensors.
2. Description of the Related Art
A storable phosphor stores part of radiation energy when exposed to radiation, and emits photostimulated luminescent light according to the stored energy when exposed to excitation light such as visible light, laser light, etc. A radiation image recording-reproducing system utilizing this storable phosphor (stimulatable phosphor) has been put to extensive practical use in the field of computed radiography (CR). In the radiation image recording-reproducing system, the radiation image information of a subject, such as a human body, etc., is temporarily recorded on a storable phosphor sheet by irradiation of radiation. The storable phosphor sheet emits photostimulated luminescent light when scanned with excitation light such as laser light, etc. The photostimulated luminescent light is detected photoelectrically by image read means constructed of photoelectric conversion elements, and an image signal carrying the radiation image information is obtained. After this image signal has been read, the storable phosphor sheet is irradiated with erasing light and emits the radiation energy remaining therein.
In the radiation image forming method, there has been proposed another method of separating the radiation absorbing function and energy storing function of the conventional stimulatable phosphor and allotting each function to two kinds of phosphors. In this method, if a phosphor having excellent radiation absorption is used as a phosphor that fulfills the radiation absorbing function, the radiation absorption coefficient can be increased. Also, if a phosphor having excellent response to photostimulated luminescence is used as a phosphor that fulfills the energy storing function, the efficiency of taking out photostimulated luminescent light can be enhanced. Furthermore, an image reading system has been proposed in Japanese Patent Application No. 11(1999)-372978. In this system, a phosphor having excellent radiation absorption absorbs radiation and is caused to luminesce when excited with light in an ultraviolet to visible region. The luminescent light is absorbed by and stored in a phosphor (storage-only phosphor) having excellent response to photostimulated luminescence. The storage-only phosphor is excited with light in an ultraviolet to visible region and is caused to emit photostimulated luminescent light. The photostimulated luminescent light is photoelectrically read in sequence by photoelectric read means, and an image signal carrying radiation image information is obtained.
The image signal obtained by the aforementioned image reading system is subjected to image processing, such as a gradation process, a frequency process, etc., suitable for image observation and reading. After these processes, the image signal is recorded on film as a visible image for diagnosis (final image), or displayed on a high-definition CRT display, so that it can be used for diagnosis. On the other hand, if the aforementioned storable phosphor sheet is irradiated with erasing light to remove residual energy, the sheet can be repeatedly used because it can store and record radiation image information again.
It has also been proposed (Japanese Unexamined Patent Publication Nos. 60(1985)-111568, 60(1985)-236354, 1(1989)-101540, etc.) that, in order to shorten the time needed to read photostimulated luminescent light, make the reader compact, and reduce costs, the radiation image information reader in the radiation image recording-reproducing system uses a line light source for irradiating line excitation light to a phosphor sheet, as an excitation light source; uses a line sensor, which includes a large number of photoelectric conversion elements arrayed along the length direction (hereinafter referred to as a horizontal scanning direction) of a line portion on the sheet irradiated with the excitation light emitted from the line light source, as photoelectric read means; and is equipped with scanning means for relatively moving one of (1) the line light source and line sensor and (2) the phosphor sheet with respect to the other in a direction substantially perpendicular to the length direction of the light-irradiated line portion (hereinafter referred to as a vertical scanning direction).
However, in the case where the aforementioned radiation image information reader uses a charge-coupled device (CCD), which has relative high quantum efficiency (about 60%) and can be reduced in size, as a photoelectric conversion element, the light collecting efficiency including the optical system for collecting photostimulated luminescent light is about 10% or less and is not enough to obtain a diagnostic image whose S/N ratio is high. The length needed for the line sensor that is used in the radiation image information reader is the same length as the width of storable phosphor sheet, about 35 to 43 cm. However, because the lengths of commercially-available line sensors are about a few tens to 100 mm, it is necessary to use a plurality of line sensors disposed on a straight line. Since the individual line sensors are packaged, the adjacent portions in the line sensors are insensitive portions that cannot receive photostimulated luminescent light. The photostimulated luminescent light emitted from the excitation-light irradiated portion corresponding to the insensitive portions is not detected. This produces artifacts in an image signal obtained.
The present invention has been made in view of the drawbacks found in the prior art. Accordingly, it is an object of the present invention to provide a radiation image information reader that is capable of enhancing light-collecting efficiency and obtaining an image whose S/N ratio is sufficiently high. Another object of the invention is to provide a radiation image information reader in which, when using line sensors that employ CCDs, etc., an image can be obtained without producing artifacts which result from the insensitive portions of the line sensors.
To achieve the objects of the present invention mentioned above, there is provided a radiation image information reader comprising:
irradiation means for irradiating excitation light in line form to a portion of a top surface of a storable phosphor sheet storing radiation image information therein;
detection means having a line detection section in which a large number of photoelectric conversion elements for detecting photostimulated luminescent light, emitted from the excitation-light irradiated line portion or a portion of a bottom surface of the sheet corresponding to the excitation-light irradiated line portion, are arrayed parallel to the excitation-light irradiated line portion;
scan means for relatively moving one of (1) the irradiation means and detection means and (2) the phosphor sheet with respect to the other in a direction differing from a length direction of the irradiated line portion; and
read means for reading an output of the detection means sequentially according to the movement and then obtaining data which constitutes a final image;
wherein the line detection section of the detection means is constituted by a plurality of line detection sections; and
wherein the photostimulated luminescent light emitted from the irradiated line portion is detected by the plurality of line detection sections.
In the radiation image information reader of the present invention, the plurality of line detection sections are arranged parallel to the excitation-light irradiated line portion.
The excitation light that is irradiated in line form from the irradiation means may be one that is emitted from a line light source itself, or one that is emitted in line form by an optical system. The excitation light may be emitted continuously, or in a pulse form that repeats emission and stop. However, it is desirable from the viewpoint of noise reduction that it be high-output pulsed light. The line light source can employ a fluorescent lamp, a cold cathode fluorescent lamp, etc., along with a slit, or employ an LED array, an LD array, a broad area laser, etc., along with a cylindrical lens, etc.
The irradiation means and the detection means may be disposed on the same side with respect to the phosphor sheet, or disposed separately on the opposite sides with respect to the phosphor sheet. In the case where they are disposed separately, a support body for the phosphor sheet needs to allow passage of photostimulated luminescent light.
In the radiation image information reader of the present invention, the irradiation means irradiates the excitation light in line form in a direction substantially normal to the phosphor sheet, and the plurality of line detection sections are disposed on both sides across the irradiated line portion in the relatively-moving direction, respectively. Also, the irradiation means can be disposed so that the excitation light is passed through a space, between adjacent line detection sections of the plurality of line detection sections, which is widest.
The line detection section includes a plurality of line sensors arrayed adjacent to one another in a direction parallel to the irradiated line portion, each line sensor having a large number of photoelectric conversion elements arrayed in line form. In that case, the detection means is disposed so that the photostimulated luminescent light, emitted from the excitation-light irradiated portion corresponding to adjacent portions in the plurality of line sensors of at least one of the line detection sections, is detected by the remaining line detection sections in which the line sensors are arrayed so that adjacent portions in the line sensors are disposed at positions shifted predetermined distances from the adjacent portions in the one line detection section in the direction parallel to the irradiated line portion. As shown in FIG. 5, for example, photostimulated luminescent light, emitted from the excitation-light irradiated portion corresponding to adjacent portions in the line sensors, can be detected by two line detection sections 20, 20xe2x80x2. A detailed description for FIG. 5 will be described later.
The line detection section of the detection means may be constituted by at least three line detection sections. In that case, the photostimulated luminescent light, emitted from the excitation-light irradiated portion corresponding to the adjacent portions, can be detected by at least two other line detection sections in the line detection means. As shown in FIG. 6, for instance, adjacent portions in three line detection sections 24, 24xe2x80x2, 24xe2x80x3 are disposed at different positions with respect to an excitation-light irradiated portion 2. With this disposition, photostimulated luminescent light, emitted from the excitation-light irradiated portion corresponding to adjacent portions in one of the three line detection sections, can be detected by the remaining two line detection sections. A detailed description for FIG. 6 will be given later.
In the radiation image information reader of the present invention, the read means may include pixel-data addition means for adding data output from the plurality of line detection sections.
In the case where the line detection section includes a plurality of line sensors arrayed adjacent to one another in the direction parallel to the irradiated line portion, and each line sensor has a large number of photoelectric conversion elements arrayed in line form, the read means may be equipped with pixel-data addition means for adding data output from at least one of the line detection sections and data output from the remaining line detection sections, with a consecutively-variable ratio. For example, as shown in FIG. 7, when photostimulated luminescent light is detected by both a first line detection section 20 and a second line detection section 20xe2x80x2, the pixel data detected by the line sensors of the line detection sections can be added with consecutive ratios shown in FIG. 7. Adding with consecutive ratios means that additions are made, for example, while a ratio of pixel data 3xe2x80x2 detected by the line sensor 3 of the first line detection section 20 and pixel data 4xe2x80x2 detected by line sensor 4 of the second line detection section 20xe2x80x2 (3xe2x80x2:4xe2x80x2) is being varied from 9:1 to 8:2 and to 7:3, assuming a ratio when the detection is performed only by the line sensor 3 to be 10:0.
In the radiation image information reader of the present invention, the storable phosphor sheet may contain a stimulatable phosphor, which absorbs light in an ultraviolet to visible region and stores the energy and further emits the stored energy as photostimulated luminescent light when excited with light in a visible to infrared region. In this case, the radiation absorbing function and energy storing function of the conventional stimulatable phosphor are separated, and a phosphor having excellent radiation absorption (radiation-absorption phosphor) and a phosphor having excellent response to photostimulated luminescence (storage-only phosphor) are used. The radiation-absorption phosphor is caused to absorb radiation and luminesce in an ultraviolet to visible region. The storage-only phosphor is caused to absorb the luminescent light and store the energy and is further caused to emit the stored energy as photostimulated luminescent light when excited with light in a visible to infrared region.
According to the radiation image information reader of the present invention, the line detection section of the detection means is constituted by a plurality of line detection sections. The photostimulated luminescent light emitted from an irradiated line portion can be detected by the plurality of line detection sections. As a result, light-collecting efficiency is enhanced, whereby an image with a higher S/N ratio can be obtained and an image diagnosis can be made with higher reliability.
According to the radiation image information reader of the present invention, the irradiation means irradiates the excitation light in line form in a direction normal to the phosphor sheet, and a plurality of line detection sections are disposed on both sides across the irradiated line portion in the relatively-moving direction, respectively. Since the line detection sections can be disposed in the direction substantially normal to the excitation-light irradiated portion of the phosphor sheet surface, light-collecting efficiency can be further enhanced.
In addition, the irradiation means can be disposed so that the excitation light is passed through a space, between adjacent line detection sections of the plurality of line detection sections, which is widest. Thus, light-collecting efficiency can be further enhanced.
In the radiation image information reader of the present invention, the detection means can be disposed so that the photostimulated luminescent light, emitted from the excitation-light irradiated portion corresponding to adjacent portions in the plurality of line sensors of at least one of the line detection sections, is detected by the remaining line detection sections in which the line sensors are arrayed so that adjacent portions in the line sensors are disposed at positions shifted predetermined distances from the adjacent portions in the one line detection section in the direction parallel to the irradiated line portion. This disposition can avoid the occurrence of artifacts. When the line detection section of the detection means is constituted by at least three line detection sections, the photostimulated luminescent light, emitted from the excitation-light irradiated portion corresponding to adjacent portions in the plurality of line sensors of at least one of the line detection sections, can be detected by at least two other line detection sections. Not only can the occurrence of artifacts be avoided, but light-collecting efficiency can also be enhanced. Thus, an image with a higher S/N ratio can be obtained.
In the radiation image information reader of the present invention, the read means can be equipped with pixel-data addition means for adding data output from a plurality of line detection sections, and it can be implemented with a simple device.
Furthermore, the read means can be equipped with pixel-data addition means for adding data output from at least one of the line detection sections and data output from the remaining line detection sections, with a consecutively-variable ratio. Therefore, data obtained at the insensitive portion of the adjacent portion in the line sensor can be prevented from being discontinuous. As a result, there is obtained a smoother image in which artifacts have been suppressed.
In the radiation image information reader of the present invention, the storable phosphor sheet can contain a stimulatable phosphor, which absorbs light in an ultraviolet to visible region and stores the energy and further emits the stored energy as photostimulated luminescent light when excited with light in a visible to infrared region. In this case, if the stimulatable phosphor having excellent response to photostimulated luminescence is utilized as a storage-only phosphor and is utilized in combination with a phosphor having excellent radiation absorption and that absorbs radiation and emits light in an ultraviolet to visible region, detection quantum efficiency, radiation absorption efficiency, photostimulated-luminescence efficiency, photostimulated-luminescence taking-out efficiency, etc., can be enhanced.